Heating, Ventilation and Air-Conditioning (“HVAC”) systems provide circulation of indoor air in enclosed environments, including buildings and structures of all kinds, vehicles, and vessels. The HVAC system's primary role is to maintain comfortable temperature and humidity, as well as good indoor air quality. In order to maintain good air quality, the circulating air should be refreshed, either by continually replacing it with fresh air from outside the enclosed environments, or by treating it for removal of unwanted contaminants that tend to form or buildup in the enclosed environments. The contaminants may also be referred to as pollutants, or substances. These pollutants may include carbon dioxide (CO2) as well as volatile organic compounds (VOCs), inorganic gases like sulfur oxides, nitrous oxides, carbon monoxide, radon and others. Particles and microorganisms also represent non-gaseous pollutants that affect indoor air quality and should be filtered or removed. Since replacement of indoor air may not provide a satisfactory solution—whether due to the thermal load it imposes on the HVAC system, or due the poor air quality of the outside air, or lack of access to outside fresh air, indoor air may be treated by means of adsorbents to remove gas pollutants.
As adsorbents collect pollutants from the air, they gradually become saturated and lose their adsorptive efficiency. In order to use adsorbents for an extended service period, it is often necessary to periodically purge them in a process known as regeneration. Regeneration may be performed by streaming a purge gas (which may also be referred to as a regeneration gas) over and/or through the adsorbent. For example, the adsorbent may be flushed with air or some other gas that has a higher temperature and/or lower partial pressure of pollutants, or by heating the adsorbent itself, whereby the pollutant molecules are carried off the adsorbent surface. Thus, adsorbents are often used in an adsorption-desorption “swing cycle” where in the adsorption, or cleaning, part of the cycle, they capture certain species of gases, and continue to do so until the adsorbent reaches saturation. During desorption, or a regeneration cycle, they release the gases which were adsorbed until they recover their original capacity and adsorption efficiency, at which point a new cycle can begin. The swing cycle can be at least one of, for example, a temperature-swing cycle, pressure-swing cycle, or a concentration-swing adsorption cycle. In some cases both temperature and concentration, or temperature and pressure, may be changed during regeneration.
The use of regenerable adsorbents for removing carbon dioxide (CO2) and VOCs in indoor air, relying on a swing cycle is an important alternative to air replacement, especially when the outside conditions make air replacement energetically costly, environmentally undesirable or otherwise impractical. However regeneration economics and performance are critical, as regeneration represents downtime for the air treatment function and energy must be consumed to flow the purge gas and release the pollutants. Finding the optimal economic and functional performance is further complicated by varying conditions of pollutant species and concentration levels, as well as temperature and flow rates.